If The Internet In Idaho Goes Out, This Is Why
The Down Detector has a real-time display of what parts of the country are currently without Internet.
Without getting extremely technical, this outage has been caused by a DDOS attack. Here's a quick explanation from Wikipedia on what that means.
In computing, a denial-of-service (DoS) attack is a cyber-attack where the perpetrator seeks to make a machine or network resource unavailable to its intended users, such as to temporarily or indefinitely interrupt or suspend services of a host connected to the Internet. Denial of service is typically accomplished by flooding the targeted machine or resource with superfluous requests in an attempt to overload systems and prevent some or all legitimate requests from being fulfilled.
According to Gizmodo, Homeland Security is in the process of figuring out who is behind today's attack.